The present invention relates generally to ophthalmic lenses suited for both distance vision and near vision, namely for correcting farsightedness.
As is well known there are two general categories of ophthalmic lenses which fulfill this requirement. First of all, there are bifocal ophthalmic lenses which have a first constant refractive power zone forming the major part of the lens and adapted for distance vision on which is locally superposed in the lower part of the lens, forming a segment, a second constant refractive power zone for near vision. Such bifocal ophthalmic lenses which are well adapted to users leading a sedentary existence not requiring frequent switching from near vision to distance vision and vice versa have the advantage of providing extensive distance vision and near viewing areas. For static vision they provide very good visual comfort.
But such bifocal lenses have the drawback of a boundary at the upper limit of the second constant refractive power zone separating the first and second constant refractive power zones which results in a power discontinuity for most such bifocal lenses when passing from the near viewing zone to the distance vision zone and vice versa, causing an annoying "jump" which is all the greater with large power differences.
The second type of ophthalmic lens which is well suited to both distance vision and near vision is the so-called progressive ophthalmic lens, i.e., an ophthalmic lens comprising a zone of progressively variable refractive power along at least one of its meridians.
In practice, the progressively variable refractive power zone covers generally the lower part of the lens for near vision whereas the upper part of the lens defines a constant refractive power zone adapted to distance vision. The progressive refractive power zone is continuously connected to the constant refractive power zone and progressively adds additional power thereto whose value measured along the principal meridian between the upper boundary of the progressively variable refractive power zone and the effective lower boundary thereof corresponds to the additional near correction necessary referred to as the addition.
Such progressive ophthalmic lenses advantageously provide a continuous power transition without any discontinuity or jump between the near viewing zone and the distance viewing zone. They therefore offer excellent dynamic viewing comfort and therefore are particularly suitable for persons whose activity requires frequent changes from distance viewing to near viewing and vice versa.
But such progressive ophthalmic lenses have various drawbacks, namely they provide, for intermediate viewing between distance and near viewing only a relatively narrow field of correct vision to either side of the principal meridian, the field of correct vision being flanked by lateral zones with greater or lesser aberrations which bothers viewing and is all the more annoying with greater power additions of the progressively variable refractive power zone.